Fishing boats and vessels are often equipped with a trolling motor for providing a relatively small amount of thrust to slowly and quietly propel the boat or vessel while the operator is fishing. The motor is typically mounted to the bow of the boat (alternatively, the motor may be mounted at other locations in the boat, for example the motor may be transom mounted at the stern of the boat) so that the thrust pulls the boat through the water.
Some existing trolling motors include mechanical cable linkages between a foot pedal or steering control and a mechanical steering system (e.g., rack and pinion) in a steering head. The operator provides the physical force for turning the lower unit of the motor via the linkages. In such systems, the rotary motion of the foot pedal or other steering control is mechanically converted to rotary motion of a prop motor or propulsion unit. Such mechanical systems, however, require relatively bulky cable linkages, and do not have the flexibility provided by electronic steering control systems. Other existing trolling motors have an electric power steering system wherein an electric steering motor is used to rotate the trolling motor lower unit to steer the boat. Such systems use either "open-loop steering control" or "closed-loop steering control" ("feedback steering"). In one open-loop system, the foot pedal assembly includes a pair of switches and when the user pushes the right (left) side of the foot pedal, the first (second) switch is closed to actuate a steering motor to cause the lower unit to turn to the right (left) for as long as a first (second) switch is held closed. In one closed-loop system, the foot pedal assembly includes a potentiometer for sensing the rotational position of the foot pedal and generating a steering command signal representative thereof, and the trolling motor includes a potentiometer for sensing the rotational position of the prop motor, and an electronic controller for controlling the steering motor based on the difference between the steering command and feedback signals.
Although both types of existing electric "power steering" systems avoid the need for bulky mechanical steering linkages, and provide flexibility through electronic control, each can be advantageous over the other in certain situations. Closed-loop systems provide the obvious advantage of acting like mechanical systems since the steer direction depends on the rotational position of the foot pedal. However, in situations in which the user wishes to make a fine adjustment to the steering direction, relationship between foot pedal position (which may have a total range of only 45 degrees, or +/-22.5 degrees) and prop position (which may have a total range of 360 degrees) may yield a high steering ratio (e.g., 8;1), which can make fine adjustments difficult to achieve. For example, if the user wants to make only a 4 degree adjustment, he needs to rotate the pedal by only 0.5 degrees. This resolution can be difficult to achieve using foot control, which is not as accurate as hand control, and can be difficult to achieve in a bouncing and pitching boat. In this situation, open-loop systems may be preferred since a user merely has to tap the left or right steering switch with his foot to cause a prop to rotate at a predetermined rate (e.g., 5 degrees per second). In the example, the user could achieve the 4 degree turn merely by tapping the left or right switch for 0.8 seconds, which is relatively easy. Thus, both open-loop and closed-loop steering systems can be disadvantageous under certain conditions.
Accordingly, there is a need for an electronic power steering system for a trolling motor wherein features of both open and closed-loop steering systems are combined to provide a single steering system having advantages of both open and closed-loop steering systems.